Chemical modification of arginine residues in alpha-bungarotoxin

Separation and structure-function studies of Taiwan cobra cardiotoxins

Six cardiotoxins (CTXs) and one cardiotoxin-like basic protein (CLBP) from Naja naja atra (Taiwan cobra) venom were separated by a SP-Sephadex C-25 column. CTXn and CTXI were well separated by eluting with ammonium acetate buffer, and the separation of CLBP from CTXIV and CTXV mixtures was achieved using sodium phosphate buffer. These findings suggest a differential interaction of CTXs with the chromatographic matrix using different buffer systems. Chemical modification studies on cationic residues of CTXI suggested that there was no single lysine or arginine residue exclusively responsible for its biological activity. Moreover, it was found that the cytotoxicity and hemolytic sites of CTXI could be dissociated by chemical modifications. It suggests the potentiality for preparing toxin derivatives in which a specific activity is retained.

Probing the structural diversities of long alpha-neurotoxins by fluorescence quenching studies

Trp fluorescence of Ophiophagus hannah neurotoxins (Oh-4, Oh-6A, Oh-7, and Oh-8) and Bungarus multicinctus (alpha-bungarotoxin was quenched by acrylamide and iodide. Acrylamide quenching studies indicated that the degree of exposure of Trp residues in the neurotoxins followed the order Oh-8 > Oh-7 > Oh-6A > Oh-4 > alpha-bungarotoxin, as did the accessibility for iodide. These results reveal that the exposed degree of Trp residues and the microenvironment surrounding Trp residues in the neurotoxins differ, even though their Trp residues and positively charged residues are located at the same or homologous positions. In contrast to unfolded Oh-4, Oh-6A, Oh-7, and alpha-bungarotoxin, unfolding of Oh-8 by reduction and S-carboxymethylation caused a notable decrease in the susceptibility of their Trp residues for iodide. These observations support the view that the side chains of Trp residues and positively charged residues in their native structure do not point toward the same spatial positions. Computer models of the neurotoxins are in good agreement with this proposition. These results elucidate why the conserved Trp residues and cationic groups do not always play the same roles in the biological activities of the neurotoxins.

Differential contribution of the conserved tyrosine residues to activity and structural stability of Ophiophagus hannah alpha-neurotoxins

Two alpha-neurotoxins, Oh-4 and Oh-7, from king cobra (Ophiophagus hannah) venom were subjected to Tyr modification with tetranitromethane. Selective nitration of Tyr4 in Oh-4 caused a slight decrease in lethal toxicity of 11% and a decrease in nicotinic acetylcholine receptor (nAchR)-binding activity of 28%, whereas nitration of Tyr4 in Oh-7 resulted in an approximately equal 60% decrease in lethality and nAchR-binding activity. When the Tyr23 in Oh-4 or Tyr22 in Oh-7 appears to be 'buried' in the toxin following further modification, the toxins lost their biological activity and conformational change concurrently. Nevertheless, the dinitrated Oh-4 retained a beta-sheet structure as revealed by CD spectra and exhibited a precipitin reaction with anti-Oh-4 sera. These results indicate that both Tyr4 and Tyr22 play a crucial role in the neurotoxicity of Oh-7, whereas intact Tyr23 is involved in the manifestation of the toxicity of Oh-4 to a greater extent. In contrast to Oh-4, the conformational stability of Oh-7 depends heavily upon the integrity of Tyr22.

Variability among the sites by which curaremimetic toxins bind to torpedo acetylcholine receptor, as revealed by identification of the functional residues of alpha-cobratoxin

alpha-Cobratoxin, a long chain curaremimetic toxin from Naja kaouthia venom, was produced recombinantly (ralpha-Cbtx) from Escherichia coli. It was indistinguishable from the snake toxin. Mutations at 8 of the 29 explored toxin positions resulted in affinity decreases for Torpedo receptor with DeltaDeltaG higher than 1.1 kcal/mol. These are R33E > K49E > D27R > K23E > F29A >/= W25A > R36A >/= F65A. These positions cover a homogeneous surface of approximately 880 A(2) and mostly belong to the second toxin loop, except Lys-49 and Phe-65 which are, respectively, on the third loop and C-terminal tail. The mutations K23E and K49E, and perhaps R33E, induced discriminative interactions at the two toxin-binding sites. When compared with the short toxin erabutoxin a (Ea), a number of structurally equivalent residues are commonly implicated in binding to muscular-type nicotinic acetylcholine receptor. These are Lys-23/Lys-27, Asp-27/Asp-31, Arg-33/Arg-33, Lys-49/Lys-47, and to a lesser and variable extent Trp-25/Trp-29 and Phe-29/Phe-32. In addition, however, the short and long toxins display three major differences. First, Asp-38 is important in Ea in contrast to the homologous Glu-38 in alpha-Cbtx. Second, all of the first loop is insensitive to mutation in alpha-Cbtx, whereas its tip is functionally critical in Ea. Third, the C-terminal tail may be specifically critical in alpha-Cbtx. Therefore, the functional sites of long and short curaremimetic toxins are not identical, but they share common features and marked differences that might reflect an evolutionary pressure associated with a great diversity of prey receptors.

Solution structure of toxin b, a long neurotoxin from the venom of the king cobra (Ophiophagus hannah)

The solution structure of toxin b, a long neurotoxin (73 amino acids and 5 disulfides) from the venom of Ophiophagus hannah (king cobra), has been determined using 1H NMR and dynamical simulated annealing techniques. The structures were calculated using 485 distance constraints and 52 dihedral angle restraints. The 21 structures that were obtained satisfy the experimental restraints and possess good nonbonded contacts. Analysis of the converged structures revealed that the protein consists of a core region from which three finger-like loops extend outwards. The regular secondary structure in toxin b includes a double and a triple stranded antiparallel beta sheet. Comparison with the solution structures of other long neurotoxins reveals that although the structure of toxin b is similar to those of previously reported long neurotoxins, clear local structural differences are observed in regions proposed to be involved in binding to the acetylcholine receptor. A positively charged cluster is found in the C-terminal tail, in Loop III, and in the tip of Loop II. This cationic cluster could be crucial for the binding of the long neurotoxins to the acetylcholine receptor.

Functional dissection of the Moloney murine leukemia virus envelope protein gp70

The envelope protein of Moloney murine leukemia virus (Mo-MLV) is a complex glycoprotein that mediates receptor binding and entry via fusion with cell membranes. By using a series of substitution mutations and truncations in the Mo-MLV external envelope surface protein gp70, we have identified regions important for these processes. Firstly, truncations of gp70 revealed that the minimal continuous receptor-binding region is amino acids 9 to 230, in broad agreement with other studies. Secondly, within this region there are two key basic amino acids, Arg-83 and Arg-95, that are essential for receptor binding and may interact with a negatively charged residue(s) or with the pi electrons of the aromatic ring on a hydrophobic residue(s) in the basic amino acid transporter protein that is the Mo-MLV ecotropic receptor. Finally, we showed that outside the minimal receptor-binding region at amino acids 2 to 8, there is a region that is essential for postbinding fusion events.

Chemical modification of cationic residues in toxin a from king cobra (Ophiophagus hannah) venom

The cationic groups of arginine and lysine residues in alpha-neurotoxin, Toxin a, isolated from king cobra (Ophiophagus hannah) venom were subjected to modification with trinitrobenzene sulfonate (TNBS) and p-hydroxyphenylglyoxal (HPG), respectively. The trinitrophenylated (TNP) derivatives of Toxin a at Lys-10, 56, or 71 showed approximately 25% residual lethality, and modifications on Lys-10 and 56 or Lys-10 and 50 resulted in a decrease of lethality by 84% and 86%, respectively. Modifications on Arg-34, 37, and 70 and Arg-34, 37, and 72 in Toxin a caused a decrease in lethality by 92% and 93%, respectively, and it almost completely lost its lethality and binding activity to nicotinic acetylcholine receptor (nAChR) when all four arginine residues were modified. These results indicate that in addition to the cationic residues on loop II (Arg-34, 37), loop III (Lys-50, 56), and the C-terminal tail (Arg-70, 72; Lys-71), Lys-10 on loop I is also related to the neurotoxicity of Toxin a.

Chemical modification of tryptophan residues in alpha-neurotoxins from Ophiophagus hannah (king cobra) venom

Two alpha-neurotoxins, Oh-4 and Oh-7, from the king cobra (Ophiophagus hannah) venom were subjected to Trp modification with 2-nitrophenylsulfenyl chloride (NPS-Cl). One major NPS derivative was isolated from the modified mixtures of Oh-4 and two from Oh-7 by HPLC. Amino acid analysis and sequence determination revealed that Trp-27 in Oh-4, and Trp-30 and Trp-26 and 30 in the two Oh-7 derivatives, were modified, respectively. Sulfenylation of Trp-27 in Oh-4 caused about 70% drop in lethal toxicity and nicotinic acetylcholine receptor-binding activity. Modification of Trp-30 in Oh-7 resulted in the decrease of lethal toxicity by 36% and binding activity by 61%. The activities were further lost when the conserved Trp-26 in Oh-7 was modified. Sulfenylation of the Trp residues did not significantly affect the secondary structure of the toxins as revealed by the CD spectra. These results indicate that the Trp residues in these two long alpha-neurotoxins may be involved in the receptor binding.